RARE-EARTH CRYSTAL GROWTH FROM THE VAPOR : Eu/Re AND Eu/W

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RARE-EARTH CRYSTAL GROWTH FROM THE VAPOR : Eu/Re AND Eu/W

A. Melmed, V. Maurice, O. Frank, J. Block

To cite this version:

A. Melmed, V. Maurice, O. Frank, J. Block. RARE-EARTH CRYSTAL GROWTH FROM THE VAPOR : Eu/Re AND Eu/W. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-47-C9-52.

�10.1051/jphyscol:1984909�. �jpa-00224387�

JOURNAL DE PHYSIQUE

Colloque C.9, suppl6ment au n012, Tome 45, d6cernbre 1984 page C9-47

RARE-EARTH CRYSTAL GROWTH FROM THE VAPOR : E U / R ~ AND E U / W

A.J. Melmed*, V. Maurice', 0. Frank and J.H. Block

F r i t z - H a b e r - I n s t i t u t d e r Max-Planck-Gesellsehaft, Famdayweg 4-6, 0-1000 B e r l i n 33, F.R.G.

R6sum6: La c r o i s s a n c e c r i s t a l l i n e Bpitaxique dlEu s u r Re e t dtEu s u r W a 6 t 6 BtudiBe au moyen de l a microscopie d16mission de champ

Blectronique. NuclBation, c r o i s s a n c e c r i s t a l l i n e e t r e l a t i o n s d 1 6 p i t a x i e s o n t comparativement d g c r i t e s e t d i s c u t 6 e s . Une valeur du t r a v a i l de s o r t i e pour un c r i s t a l dlEu ?A p o l y f a c e t e s e s t donn6e.

Abstract: E p i t a x i a l c r y s t a l growth of Eu/Re and Eu/W has been s t u d i e d using F i e l d Emission Microscopy. Nucleation, c r y s t a l growth and e p i t a x i a l r e l a t i o n - s h i p s a r e comparatively d e s c r i b e d and d i s c u s s e d , and a v a l u e of t h e e l e c t r o n work f u n c t i o n f o r p o l y - f a c e t t e d Eu has been determined.

I. I n t r o d u c t i o n

I n t h e l i t e r a t u r e on modern, c l e a n - s u r f a c e s t u d i e s of metal e p i t a x y on m e t a l s , it seems t h a t no e f f o r t has been made t o use s u b s t r a t e s w i t h t h e HCP s t r u c t u r e / I / . Experiments using such s u b s t r a t e s could y i e l d i n t e r e s t i n g a d d i t i o n a l information about t h e i n f l u e n c e of s u b s t r a t e atomic s t r u c t u r e on t h e e p i t a x i a l growth p r o c e s s , e s p e c i a l l y i f t h e n a t u r a l s t r u c t u r e of t h e e p i t a x e d c r y s t a l l a y e r is o t h e r t h a n HCP. Also, t h e use of curved s u b t r a t e s could provide information about t h e p r e f e r r e d s i t e s , i f any, f o r n u c l e a t i o n of c r y s t a l s . T h e r e f o r e , we have i n v e s t i g a t e d t h e e p i t a x i a l c r y s t a l growth of europium, a BCC m e t a l , on rhenium, a n HCP m e t a l , u s i n g F i e l d Emission Micros- copy (FEM)/2/. This technique e n a b l e s very c a r e f u l c o n t r o l of t h e e p i t a x i a l g y s t a l growth parameters under u l t r a h i g h vacuum c o n d i t i o n s /3/. Eu ( a = 4.58 A) was chosen because it has no known a l l o t r o p i c t r a n s f o r m a t i o n s and is a low m e l t i n g temperature metal. Re ( a = 2.76, c = 4.46 A) was s e l e c t e d a s sub- s t r a t e m a t e r i a l i n an a t t e m p t t o minimize i n t e r d i f f u s i o n and a l l o y formation.

Another s e t of experiments f o r Eu on t u n g s t e n was performed i n o r d e r t o answer some a d d i t i o n a l q u e s t i o n s which a r o s e d u r i n g t h e course of t h e Eu/Re e x p e r i - ment.

11. Experiment

The g r e a t chemical r e a c t i v i t y and r e l a t i v e l y h i g h vapor p r e s s u r e (PV =

i'x10-~ Pa a t bakeout temperature of 625 ^{O K ,} f o r example) of Eu made n e c e s s a r y some s p e c i a l experimental procedures. The Eu was cleaned w i t h cyclohexanol and d r i e d w i t h e t h e r . I t was t h e n c u t i n s m a l l p i e c e s (one m i l l i m e t e r d i a m e t e r ) under an argon atmosphere. These Eu p i e c e s were t h e n put i n t o a s l i d i n g g l a s s t u b e i n s i d e a sidearm which was added t o a conventional g l a s s f i e l d emission microscope /2,3/. During t h e bakeout, t h e temperature i n t h e p a r t of t h e oven c o n t a i n i n g t h e sfdearm w i t h Eu p i e c e s was kept lower (455 K) t h a n t h e p a r t c o n t a i n i n g t h e main body of t h e microscope (625 K). The s l i d i n g g l a s s t u b e was

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'permanent address: S u r f a c e Science D i v i s i o n , N a t i o n a l Bureau o f S t a n d a r d s , Gaithersburg, MD 20899 U.S.A.

+permanent address: L a b o r a t o i r e de Physico-Chimie des S u r f a c e s , E.N.S.C.P., U n i v e r s i t 6 P a r i s V I , France.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984909

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l a t e r used t o t r a n s p o r t t h e Eu p i e c e s from t h e sidearm i n t o a degassed T a - f o i l b u c k e t , which was h e a t e d by e l e c t r i c a l c u r r e n t t h r o u g h W l e a d s , t o e v a p o r a t e Eu o n t o t h e m e t a l s u b s t r a t e . The f i e l d e m i t t e r , a t t a c h e d t o a W l o o p , was

t h e r m a l l y c l e a n e d and t e m p e r a t u r e was c o n t r o l l e d by a n e l e c t r o n i c t i p tempera- t u r e c o n t r o l l e r . The base p r e s s u r e , a ~ h i e v e ~ w i t h a s p u t t e r - i o n pump and a t i t a n i u m g e t t e r b u l b , s u f f e r e d somewha from t h e l e s s t h a n o p t i m a l bakeout p r o c e d u r e , and u s u a l l y was about 4x10-'Pa. Thus t h e Eu c r y s t a l s grown may i n c l u d e some contamination. A l l o f t h e f i e l d e m i s s i o n o b s e r v a t i o n s and photographs r e p o r t e d were made a t o r n e a r 78 K , m a i n t a i n e d by l i q u i d n i t r o g e n c o o l i n g o f t h e t i p s u p p o r t l e a d s .

111. R e s u l t s and Comments

I n o r d e r t o determine t h e approximate growth c o n d i t i o n s p r i o r t o t h e e p i t a x i a l c r y s t a l growth e x p e r i m e n t s , we measured t h e o n s e t

t e m p e r a t u r e f o r m u l t i l a y e r s u r f a c e d i f f u s i o n ( i . e , when s u r f a c e d i f f u s i o n w i t h a s h a r p boundary o c c u r r e d o v e r t h e e n t i r e t i p a f t e r exposure t o Eu vapor a t 80 K ) . The r e s u l t s were 440 K f o r Eu/Re and 460 K f o r Eu/U ( f o r z e r o - f i e l d c o n d i t i o n s ) .

The u s u a l procedure, a d o p t e d a f t e r s e v e r a l a t t e m p t s , used t o grow c r y s t a l s was t o f i r s t n u c l e a t e them a t t h e maximum p o s s i b l e s u b s t r a t e tempera- t u r e f o r a given e v a p o r a t i o n r a t e , i n o r d e r t o minimize t h e number o f

n u c l e a t i o n s i t e s / 4 / . T h i s was f o l l o w e d by a l o w e r i n g o f t h e s u b s t r a t e t e m p e r a t u r e , t o a l l o w t h e c r y s t a l t o grow l a r g e r . For t h e Eu/Re system, c r y s t a l growth was a c h i e v e d i n t h e 445 K - 510 K t e m p e r a t u r e r a n g e and r e s u l t e d i n t h e f o l l o w i n g f e a t u r e s . A pre-coverage o f more t h a n one monolayer was n e c e s s a r y f o r c o n d i t i o n s under which n u c l e a t i o n followed by c r y s t a l growth o c c u r r e d . T h i s i n d i c a t e s a pseudo-Stranski-Kratanov (SKI mode o f c r y s t a l growth, under c o n d i t i o n s where thermodynamical e q u i l i b r i u m is n o t a c h i e v e d / I / . Due t o t h e s u r f a c e topography o f t h e Re s u b s t r a t e and i t s h i g h symmetry (Fig.

l a ) , we always observed s e v e r a l n u c l e i l o c a t e d on e q u i v a l e n t s u b s t r a t e s i t e s (Fig. 2b). These n u c l e a t i o n s i t e s u s u a l l y were t h e edges o f t h e (101) and (100) p l a n e s and t h e e q u i v a l e n t o n e s by symmetry, i . e . r e s p e c t i v e l y ( 1 0 7 ) , (Oil), ( 0 1 i ) and (010). O c c a s i o n a l l y , n u c l e a t i o n was observed a t t h e s t e p s around t h e b a s a l p l a n e , (001).

We were never a b l e t o grow a s i n g l e Eu c r y s t a l l a y e r which extended o v e r most o f t h e o b s e r v a b l e Re s u r f a c e . I n f a c t , a c o n s i s t e n t l i m i t a t i o n o c c u r r e d i n t h e l a t e r a l growth; t h e I001)Eu p l a n e s never f u l l y developed, p r o b a b l y due t o a s t e r i c mismatch a t t h e Re s u r f a c e r e g i o n s where t h e (001}Eu p l a n e s o c c u r r e d . We always observed e i t h e r s e v e r a l i n c o m p l e t e c r y s t a l s i n d i f f e r e n t o r i e n t a t i o n s (Fig. l c ) o r a s i n g l e incomplete c r y s t a l (Fig. I d ) . The most commonly found e p i t a x i a l r e l a t i o n s h i p s were: (11O)Eu / / (?Ol)Re w i t h [ l i l ] E u / / C0101Re o r [TI 1 IEu / / C0101Re o r COO1 lEu / / [OlOIRe. We a l s o observed.

b u t r a r e l y : (11O)Eu / / (100)Re w i t h [ i l l ]EU / / [OlO]Re o r [OOllEu / / [OlOIRe. I t s h o u l d be noted t h a t < I l l > and <010> a r e close-packed atom rows r e s p e c t i v e l y i n t h e BCC and HCP l a t t i c e s .

A s t r a n g e e x c e p t i o n t o t h e s e o b s e r v a t i o n s sometimes o c c u r r e d i n t h e (101) - (102)Re r e g i o n , on ( 2 0 3 ) , where Eu c r y s t a l l i t e s grew having a n appar- e n t l y FCC s t r u c t u r e w i t h a c a p s h a p e of s m a l l e r d i a m e t e r t h a n t h e s u b s t r a t e . These c r y s t a l l i t e s c o u l d be o b s e r v e d e i t h e r a l o n e (Fig. l e ) o r w i t h an incom- p l e t e BCC c r y s t a l ( F i g . I f ) . They had t h e i r incomplete (111) p l a n e s growing on t h e (101)Re p l a n e w i t h C l i o l ~ u / / [OlOlRe ( n o t e t h a t <110> a r e close-packed atom rows i n t h e FCC l a t t i c e ) . We never could grow t h e "FCC11 c r y s t a l l i t e s l a r g e r t h a n t h o s e shown i n t h e f i g u r e s . Post-growth a n n e a l i n g a t growth t e m p e r a t u r e f o r a b o u t 10 s e c o n d s ( w i t h no e l e c t r i c f i e l d ) caused a transforma- t i o n t o t h e normal BCC form.

Fig. 1 FEM p a t t e r n s f o r Eu/Re a t 78 K.

a ) Clean Re, b ) Pre-coverage of Eu/Re, c ) P o l y c r y s t a l l i n e Eu l a y e r , d ) S i n g l e , incomplete Eu c r y s t a l , e ) ttFCCm c r y s t a l l i t e , f ) BCC-"FCCvt b i - c r y s t a l .

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F i g . 2 FEM p a t t e r n s f o r Eu/W a t 78 K.

a ) C l e a n W , b ) Eu p r e - c o v e r a g e and n u c l e i , c ) Eu c r y s t a l as-grown, d ) Eu c r y s t a l a f t e r a n n e a l i n g f o r a b o u t 1 s e c o n d a t c r y s t a l growth

t e m p e r a t u r e .

I n a n a t t e m p t t o u n d e r s t a n d t h e s e p a r t i c u l a r f e a t u r e s o f t h e Eu/Re c r y s t a l s , we d i d a comparison s e t o f e x p e r i m e n t s f o r Eu/W, aiming t o s e e whether a change of t h e s u b s t r a t e (BCC i n s t e a d o f HCP) would modify o u r o b s e r v a t i o n s c o n c e r n i n g t h e Eu c r y s t a l s . We grew t h e Eu/W c r y s t a l s i n t h e 520 K - 635 K t e m p e r a t u r e r a n g e u s i n g t h e same method a s f o r Re.

S i m i l a r t o Eu/Re, pre-coverage was n e c e s s a r y t o a c h i e v e n u c l e a t i o n , i n d i c a t i n g f o r t h a t system, a l s o , a pseudo-SK c r y s t a l growth mode. N u c l e a t i o n o c c u r r e d a t t h e edges of t h e {{OOlIW p l a n e s , e i t h e r towards t h e {112) p l a n e s o r towards t h e {001] p l a n e s ( F i g . 2a and F i g . 2b). C o n t r a r y t o t h e Re r e s u l t s , it was p o s s i b l e on W c o n s i s t a n t l y t o n u c l e a t e two o r t h r e e c r y s t a l s and t o

c o n t i n u e t h e growth a t a lower t e m p e r a t u r e i n o r d e r t o f i n a l l y a c h i e v e a l a r g e s i n g l e Eu c r y s t a l w i t h f u l l y developed {0011 p l a n e s ( F i g . 2 c ) . G e n e r a l l y , under t h e c o n d i t i o n s f o r g.&wth o f a l a r g e Eu c r y s t a l , t h e s u b s t r a t e tempera- t u r e was low, s u c h t h a t t h e major low i n d e x c r y s t a l p l a n e s , s p e c i f i c a l l y {011]

and {OOl}, were e n l a r g e d compared t o t h e i r s i z e s i n c r y s t a l s annealed a t h i g h e r t e m p e r a t u r e s /2/. That is, t h e Eu c r y s t a l s were more p o l y h e d r a l t h a n t h e n e a r l y - h e m i s p h e r i c a l s h a p e o b t a i n e d by h i g h t e m p e r a t u r e a n n e a l i n g / 2 / . Also t h e {I121 p l a n e s were a b s e n t , p o s s i b l y due t o e n l a r g e d { I l l } p l a n e s .

Post-growth h e a t i n g , , a t t e m p e r a t u r e s up t o t h e growth t e m p e r a t u r e , however.

r e s u l t e d i n t h e development of t h r e e small, r e l a t i v e l y h i g h work f u n c t i o n ( t h a t is, d a r k ) c r y s t a l p l a n e s n e a r t h e p o s i t i o n s o f { I 1 2 1 p l a n e s . F u r t h e r h e a t i n g i n c r e a s e d t h e s i z e of t h e s e new p l a n e s and produced d a r k l i n e s c o n n e c t i n g t h e t h r e e p l a n e s . F u r t h e r h e a t i n g n e x t caused a s m a l l d a r k {111} p l a n e t o a p p e a r , and sometimes t h e dark l i n e developed i n t o c h a i n s of s e v e r a l s m a l l d a r k

p l a n e s . U s u a l l y , t h e {I121 p l a n e s a l s o appeared. These e f f e c t s a r e t o be s e e n i n t h e micrographs of F i g . 2d, and a r e p o s s i b l y due t o c o n t a m i n a t i o n by carbon.

I n t h i s r e g a r d , i t s h o u l d be n o t e d t h a t t h e i n i t i a l d e g a s s i n g o f t h e Eu p i e c e s caused carbon c o n t a m i n a t i o n o f t h e W f i e l d e m i t t e r and a l s o t h e R e f i e l d e m i t t e r . I n b o t h c a s e s we found t h a t t h e l a s t o b s e r v a b l e t r a c e s of carbon were more e a s i l y removed by d o s i n g t h e e m i t t e r w i t h Eu, t h a n by s i m p l y h e a t i n g t h e

e m i t t e r s i n vacuum. Thus, Eu a p p e a r s t o be a n e f f i c i e n t s c a v e n g e r f o r s u r f a c e carbon.

The most commonly found o r i e n t a t i o n , of t h e Eu/W c r y s t a l s , was:

(1IO)Eu / / (1lO)W w i t h [OOlIEu // [001]W. We a l s o observed, b u t r a r e l y , (11O)Eu / / (110)W w i t h [lT2]Eu / / CT121W o r [ I ~ o ] E u / / [1T2]W o r i t s e q u i v a l e n t C 1 7 0 1 ~ u / / ClT21W.

The Eu/W c r y s t a l s provided a n o p p o r t u n i t y t o determine a v a l u e f o r t h e a v e r a g e e l e c t r o n work f u n c t i o n , from t h e measured f i e l d - e l e c t r o n c u r r e n t vs.

v o l t a g e / 2 / . Assuming t h a t t h e a v e r a g e r a d i u s of c u r v a t u r e f o r t h e Eu

p o l y c r y s t a l l i n e l a y e r was t h e same a s t h e W s u b s t r a t e , t h i s r e s u l t e d i n a work f u n c t i o n v a l u e o f 2.2 eV. Most l i k e l y t h i s is a n upper l i m i t , a s t h e Eu r a d i u s is probably l a r g e r t h a n t h a t of t h e W. The r e s u l t a g r e e s w i t h t h e lower l i m i t o f an e a r l i e r photoemission s t u d y /5/.

I n t h e Eu/W s e t of e x p e r i m e n t s , we observed f o r a l l t h e o r i e n t a t i o n s found, t h a t t h e Eu c r y s t a l s grew l a t e r a l l y a c r o s s t h e e n t i r e e m i t t e r apex, i n c l u d i n g development of t h e i r [ l o o } p l a n e s . It seems, t h e r e f o r e , t h a t f o r t h e Eu/Re system, t h e symmetry ( l a r g e number of e n e r g e t i c a l l y e q u i v a l e n t s i t e s of n u c l e a t i o n ) and topography of t h e s u b s t r a t e (roughness of t h e a r e a s where t h e [OOI}Eu p l a n e s s h o u l d grow) a r e mainly r e s p o n s i b l e f o r t h e i n a b i l i t y o f t h e Eu c r y s t a l s t o grow completely. An e p i t a x i a l c r y s t a l growth s t u d y f o r a n o t h e r BCC/HPC system s h o u l d , i n t h a t r e g a r d , g i v e more i n f o r m a t i o n .

The f a c t t h a t no Eu "FCC" c r y s t a l l i t e appeared on W i n d i c a t e s t h a t t h e topography o f t h e Re s u b s t r a t e i n t h e a r e a where s u c h c r y s t a l l i t e s appeared ( ( 1 0 1 ) - ( 1 0 2 ) ) is probably r e s p o n s i b l e f o r t h e o b s e r v a t i o n s of t h i s f e a t u r e i n

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t h e c a s e o f Eu/Re c r y s t a l s . A F i e l d I o n Microscopy (FIM) s t u d y o f t h e a t o m i c s t r u c t u r e f o r a t h e r m a l l y c l e a n e d Re s u r f a c e may p r o v i d e keys f o r t h e

u n d e r s t a n d i n g o f t h i s c u r i o u s problem.

R e f e r e n c e s

1. See, f o r example, E. Bauer. Appl. S u r f a c e S c i . 11/12 (1982) 479.

2. E. W. MUller, Ergebrl. e x a k t . Naturw. J' 2 (1953) 290.

3. S e e , f o r example, A. J. Melmed, J. Appl. Phys.

36

4. A. Ciszewski and A. J. Melmed, J. C r y s t a l Growth, t o be p u b l i s h e d . 5. D. E. Eastman, Phys. Rev. B 2 (1970) 1.